Don-Chung Liu

Winter spatial distribution of fish larvae assemblages relative to the hydrography of the waters surrounding Taiwan

The aim of this study was to investigate the species composition and distribution of fish larvae in relation to hydrographic conditions in the waters surrounding Taiwan Island (TI) in February 2003. In total, 242 kinds of fish larvae belonging to 127 genera and 75 families were recognized. Among these, 109 taxa were identified to the family or genus level, others to the species level. The 12 predominant types, which constituted 71% of the total fish larvae, were Engraulis japonica, Scomber sp., Diaphus spp., Benthosema pterotum, Carangoides ferdau, Embolichthys mitsukurii, Maurolicus sp., unidentified Myctophidae, Gonostoma gracile, Trichiurus lepturus, unidentified Gobiidae, and Myctophum asperum. The distribution of fish larvae showed a clear association with water masses around TI, with higher abundances and lower species richness northwest of TI where the China Coastal Current prevails, and lower abundances and higher species diversity east of TI where the Kuroshio Current dominates. Cluster analysis distinguished three station groups and four species groups, and the distribution patterns of fish larvae also corresponded to hydrographic conditions. The total abundances of fish larvae and eight of the 12 predominant taxa showed significant and positive correlations with zooplankton abundance, which suggests that food source might be a key factor determining the abundance and distribution of fish larvae during the winter.

DIEL OSCILLATIONS IN SAILFISH VERTICAL MOVEMENT BEHAVIOR IN THE EAST CHINA SEA

A sailfish (Istiophorus platypterus) was tagged with a pop-up satellite archival tag off the eastern coast of Taiwan and moved in a northerly direction to the East China Sea, where the tag popped-up after 160 days. The total linear displacement was 550 km from deployment to pop-up location and all movements were confined to the East China Sea. After the primarily southward movement during first two months at-liberty, the sailfish changed course after September and began to swim in a northerly direction paralleling the Kuroshio Current. During these horizontal movements, the tagged animal exhibited diel oscillations in its vertical diving behavior. On 22 days of the entire 160 days-at-liberty, the sailfish dove to depths deeper than 100 m. The sailfish spent ＞85% of its time in the upper uniformly mixed layer above ~50 m, but made more extensive vertical movements during the daytime ( x = 32.2 m ± 34.5 SD) than nighttime ( x = 9.5 m ± 16.7 SD). Depths and ambient water temperatures visited ranged from 0 to 153 m and 29.7°C to 17.8°C, respectively. The depth distribution appeared to be limited by ~6°C to 8°C changes in water temperature (Delta T) relative to sea surface temperature.

Summer Assemblages of Ichthyoplankton in the Waters of the East China Sea Shelf and Around Taiwan in 2007

This study investigated the relationships between assemblages of larval fish and hydrographic features in the East China Sea (ECS) and seas surrounding Taiwan during the summer of 2007. A survey was conducted from 2007 July 1 to 2007 July 11 in the ECS and from 2007 July 4 to 2007 July 16 in the waters surrounding Taiwan. A total of 12,670 larval fishes belonging to 95 families, 163 genes, and 189 species were identified. Engraulis japonicus and Sillago japonica were the 2 dominant species, accounting for 56.15% and 6.66% of the fish larvae samples. The cluster analysis results showed that the fish larval distribution patterns corresponded to hydrographic conditions. The results were also used to identify the following larval assemblages: the Kuroshio assemblage, coastal assemblage, Changjiang diluted water (CDW) / Yellow Sea mixed water assemblage, and Taiwan Strait water assemblage. The spatial distribution results showed a clear association between high abundances of larval fishes and shallow water areas near the ECS coast, and between low abundances and offshore Kuroshio water. The canonical correlation analysis results indicated that abundances of E. japonicus and S. japonica were strongly and positively correlated with chlorophyll a, but negatively correlated with salinity. These associations suggest that food sources and CDW might be crucial factors that determine the abundance and distribution of larval fishes during the summer.

The Kuroshio variations from satellite-derived sea surface temperature and Argos satellite-tracking Lagrangian drifters

This study aimed to identify the spatial and temporal variations of the Kuroshio using satellite-derived Sea Surface Temperature (SST) data and a total of 585 Argos satellite-tracking Lagrangian drifters during 1987–2006 in the northwestern Pacific Ocean. The threshold of high surface velocity, more than 75 cm s–1, was used to determine the location of the Kuroshio Current and found that its width ranged from 50 to 100 km. Moreover, this study first conducted a synoptic view for three predominant regions of the Kuroshio, including the area east of Taiwan, the area east of the East China Sea (ECS) and the area south of Japan. We found that the Kuroshio flows at a three-step increase in speed from upstream to downstream. The maximum mean surface velocity, 153 cm s–1, occurred to the south of Japan. Furthermore, the higher surface velocity of the Kuroshio was usually during the summertime (from April to September). In addition, the four-cruise CTD (Conductivity, Temperature, Depth)-transect data observed in January 2005 were collected to identify and validate the path of the Kuroshio axes. The results showed that the measure of 17.5°C at a depth of 200 m could be regarded as an indicator of the Kuroshio axis, corresponding with the maximum surface velocities of the Argos drifters. The monthly satellite-derived SST values of the Kuroshio were usually more than 10°C, so it was difficult to find a specific isotherm to express the location of the Kuroshio. However, extensive Kuroshio fronts derived from satellite SST data were found in this study. These Kuroshio fronts, associated with high SST gradients, were usually located on the boundary between the warm water of the Kuroshio and the colder water of the shelf, corresponding with the boundary of high surface velocity of the Argos drifters. Consequently, this study suggests that these Kuroshio fronts can be regarded as an effective indicator of the Kuroshio paths and its main axes.